A side light system, such as system disclosed in JP-A-SHO 63-62104, has been broadly applied as a means to illuminate a liquid crystal display. The advantages of a side light system is that it can be made thin and can illuminate the display or board uniformly. In the side light system, halftone dots are printed on one surface of a transparent substrate having a certain thickness such as an acrylic plate, and a light from a light source such as a cold cathode ray tube is applied to the substrate through the edge of the substrate. The applied light is uniformly dispersed by the halftone dot print, and a uniform brightness can be obtained across the surface of a display. In other systems, the light source is provided directly in back of the display.
In either such light system, a reflective optical element or reflector must be provided on the back surface of the transparent light guiding plate in order to prevent light from escaping through the back surface. This reflector must be thin and must have a high reflectance property. Although a metal deposited layer such as one disclosed in JP-A-SHO 62-169105 or a white synthetic paper such as one disclosed in JP-A-SHO 63-62104 has been used as the reflector, the deposited layer is expensive and the synthetic paper cannot produce a sufficient reflectance. Accordingly, in practice, a white polyester film in which a white pigment such as titanium oxide is added, such as one disclosed in JP-A-HEI 2-269382, has been used as the reflector. However, although the reflectance of the reflector can be increased to some extent by using such a white polyester film whitened by adding a pigment such as titanium oxide, the increase of the reflectance is limited to an insufficient level. Recently voided polyester films have been used, such as the ones disclosed in U.S. Pat. No. 5,672,409, as the reflector. The voided film described offers high reflectance in a broader range of wavelengths.
Although the reflectance of the reflector described in U.S. Pat. No. 5,672,409 was high (greater than 94%) it was achieved at a thickness greater than 150 μm. A survey of the most widely used commercial films for reflectors indicated that none had reflectance above 93% with a thickness less than 150 μm (see Table 1). It is desirable to make the reflector element as thin as possible in a display so as to minimize the entire display thickness. This is especially true in displays used in cell phones or PDA's (personal digital assistant) where the reflector display is desirable at under 150 μm thickness but maintain high reflectance, greater than 93%.
The reflector described in U.S. Pat. No. 5,672,409 has high average reflectance from 330-380 nm. Although this is claimed as an advantage, in practice the elimination of light from 200 to 400 nm is desirable as this light can be damaging to the liquid crystal polymer in the display. This will become more of a problem as the other optical elements in the display are simplified, a trend in the industry. Much of the harmful UV light, 200 to 400 nm, is currently absorbed by the other optical elements in current displays but will likely not be the case in future more simplified screen designs, especially for larger television displays Therefore, an optical element or reflector is required that can achieve high reflectivity, greater than 93% in the visible wavelengths at a range of thicknesses, especially below 150 μm. It is further required that reflectors be able to minimize reflectance at wavelengths from 200 to 400 nm.